The invention relates to a rotary variable differential transformer and, in particular, a rotary variable differential transformer which may be used as a non-contacting sensor to determine angular position of an output shaft of, for example, a valve actuator.
Maintaining the accurate positioning of an actuator output shaft requires the use of a feedback device to sense the output shaft position and provide a signal to the actuator control electronics. The accuracy of the feedback signal is affected by the linking method between the sensor and the output shaft, and the accuracy of the sensor.
The basic elements of an actuator system having a feedback device include an actuator output shaft, a position sensing device, a physical link between the output shaft and the position sensing device and actuator control electronics. The physical link between the output shaft and the position sensing device typically depends on the type and location of the position sensing device. If the sensing device is located in close proximity to the output shaft (one inch or less), the link can be optical, capacitive, magnetic, or electrical. If the sensing device is not located in close proximity, the link can be optical, or mechanical, using an extension of the shaft to create the effect of close proximity. Typically, a mechanical extension is used as the physical link between the actuator output shaft and the position sensor for reasons related to ease of manufacture, maintenance and reliability.
Applications using optical sensor devices are technology limited with regard to accuracy, tend to be expensive, are sensitive to environmental conditions and can be corrupted by opaque contamination. Applications using electrical sensor devices, typically potentiometers, can be relatively inexpensive and provide good accuracy but tend to provide poor reliability. Electrical sensor devices are also are subject to wear, corrosion, vibration, and other problems. Capacitive sensor devices can have good accuracy, but are sensitive to vibration, environmental conditions and contaminants.
Prior art devices for sensing the angular position include apparatus having two pairs of inductive coils mounted with a rotor. A magnetically permeable member comprises the rotor which is located between the pairs of coils and is rotatable about an axis perpendicular to the planes of the end faces of the coils. When the rotor rotates, the inductance of one coil of each pair increases while the inductance of the other coil of each pair decreases to provide an indication of rotational position of the rotor. Other prior art devices disclose a rotary differential transformer comprising a stator assembly having two circumferentially surrounding secondary coils and a single primary coil wound over both of the secondary coils. A rotor comprising a ferromagnetic core which is a hollow cylindrical section is positioned for rotation within the coil form. The coil form includes magnetic elements in specific locations that combine with the ferromagnetic core to form three distinct flux loops depending upon the rotational position of the rotor relative to the stator assembly. These prior art devices, however, can be difficult to manufacture and tend to suffer from poor reliability and poor accuracy due to their structural designs which are susceptible to excessive wear, corrosion, vibration and other problems.
Accordingly, a need remains for a reliable and accurate position sensing apparatus having improved manufacturability (e.g. machinability and cost effectiveness) and improved operating performance in terms of strength, durability, reduced distributed capacitance (i.e. improved bandwidth) and improved position signal output.
The present invention provides a rotary variable differential transformer (RVDT) apparatus. The apparatus may be used as a non-contacting sensor for determining the angular position of a shaft connected to the rotor of the RVDT apparatus. The apparatus is designed to reliably provide an accurate shaft position signal which can be transmitted to actuator control electronics. The apparatus is also designed to have a long service life and a high tolerance of a wide range of environmental conditions. The apparatus is easily manufactured and designed to be readily accessible for maintenance.
Briefly stated, the present invention comprises a rotary variable differential transformer apparatus which includes a stator support structure and a first and second pair of magnetically permeable stator elements. The first and second pair of stator elements are supported by the stator support structure. First and second connecting members are used to connect the first pair of stator elements, and the second pair of stator elements respectively. Circuitry for generating a position indicator signal, including an array of planar coils is formed on a multi-layer printed circuit board. The circuit board containing the circuitry is coupled to the stator elements and fastened to the stator support structure using fasteners. A rotor is rotatably mounted within the stator support structure and substantially surrounded by the first and second pairs of magnetically permeable stator elements. A magnetic flux conducting element is fastened to the rotor. The rotor, when mounted within the stator support structure, magnetically couples each of the first pair of stator elements and the second pair of stator elements via the conducting element in a manner which varies in proportion to the angular position of the rotor.